The invention relates to a data transmission method in a wireless telecommunications system wherein the user signals are transmitted in a given channel and a maximum delay has been defined for said system, and wherein an outer coding and an inner coding are sequentially performed to the signals to be transmitted, and wherein a first interleaving is performed to a signal after the outer coding, the interleaving length of said first interleaving being chosen within the scope of the maximum delay defined for the system.
As well known, the channel used for transmitting signals in telecommunications connections causes interference in telecommunications. This occurs independently of the physical form of the channel, whether the channel is a radio connection, an optical fibre or a copper cable, for instance.
In order to reduce the channel-caused interference and its effects and to ensure a more reliable transmission connection, a digital signal is coded. A typical coding method used in cellular radio applications is convolution coding, which is well suited to a fading channel. In a typical radio channel between a base station and a mobile station, errors occur in bursts, in other words they are unevenly distributed in time domain. This is the reason for using in cellular radio systems interleaving, which aims at changing the bursty errors caused by a channel to separate errors independent of each other. In a known interleaving system used in the CDMA systems, for example, the coded binary symbols are grouped into blocks in which the order of the bits is changed before transmission. This method can be called bit interleaving.
Wireless data transmission systems have conventionally been used only to transmit speech. The growth in the number of different services to be transmitted means, particularly in wireless systems, that the system must be able to transmit different capacity signals, such as speech , for instance, at a data rate of 8 kbit/s and data at a rate of 64 kbit/s, over a radio path. The data transmission system should thus be able to operate effectively in an environment where transmissions of several different data rates, quality standard requirements and service types are transmitted. Services of different type tolerate delays of different length, and maximum permissible delays corresponding to a particular service are set for the system, and the interleaving, for instance, also has to be performed within the scope of the maximum delays.
To enable transmission of high data rate signals particularly, coding methods have been developed in which two sequential codings are performed, a so called outer coding first, and a so called inner coding next. As a typical outer coding can be Reed-Solomon coding, for instance, whereupon a signal is usually formed to be frame-structured, and as a typical inner coding can be some convolution coding. Owing to the above reasons, interleaving has previously been employed in connection with this method. Interleaving has been performed in such a manner that the interleaving length of the interleaving performed after the outer coding has been chosen on the basis of the maximum system delay. The interleaving length of the interleaving subsequent to the inner coding has been determined by the length of the frame structure. This has aimed at minimizing the system delay.
The problem in the above arrangement is that the benefit gained from interleaving cannot be utilized in full, since the interleaving length of the interleaving has been determined by the length of the frame structure, which typically is substantially shorter than the maximum system delay.
An object of the invention is thus to provide a method and a system solving the above problem. This is achieved with a method described in the introduction, the method being characterized in that a second interleaving is performed to the signal after the inner coding, the interleaving length of said second interleaving being substantially equal to the interleaving length of the first interleaving.
The invention further relates to a data transmission system wherein user signals are transmitted wirelessly in a given channel and a maximum delay has been defined for said system, said system comprising means at the transmitting end for performing sequentially an outer coding and an inner coding to the signals to be transmitted, means for performing a first interleaving to a signal after the outer coding, the interleaving length of said first interleaving being chosen within the scope of the maximum delay determined for the system. The system of the invention is characterized in that the system comprises means for performing a second interleaving to the signal after the inner coding, the interleaving length of said second interleaving being substantially equal to the interleaving length of the first interleaving.
Several advantages are achieved by the method and system of the invention. The invention is based on the fact that the interleaving subsequent to the inner coding does not have to be dependent on the length of the system frame structure, but the interleaving can be equal in length to the interleaving subsequent to the outer coding. In the solution of the invention, the system performance increases on account of the improved time diversity, which is important particularly when high capacity connections are involved. If a maximum system delay is 220 ms, for example, and the length of the system frame is 10 ms, the interleaving subsequent to the outer coding has previously been performed at a 100 ms interleaving length and the interleaving subsequent to the inner coding has been performed at a 10 ms interleaving length. The total delay caused by interleaving is thus 100 ms+10 ms, i.e., 110 ms, which leaves 20 ms for the other delays. In the solution of the invention, each interleaving is performed at a 100 ms interleaving length, whereby the time diversity of the interleaving can be utilized considerably more than before. Contrary to the prior art, in the solution of the invention the total system delay does not increase even though the interleaving length is increased.
An essential advantage of the invention is also that the total system delay can be decreased by the solution of the invention without impairing the performance.